1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a liquid crystal display device using an OCB (Optically Compensated Bend) technique, which can realize a wide viewing angle and high responsivity.
2. Description of the Related Art
Liquid crystal display devices have been applied to various fields, taking advantage of their features of light weight, small thickness and low power consumption.
In currently widely marketed twisted nematic (TN) type liquid crystal display devices, liquid crystal molecules with optically positive refractive-index anisotropy are oriented with a nearly 90° twist between a pair of substrates. In the TN liquid crystal display device, the optical rotating power of incident light on the liquid crystal layer is adjusted by controlling the twisted orientation of liquid crystal molecules. The TN liquid crystal display device can be relatively easily manufactured, but the viewing angle is narrow and the responsivity is low. Thus, the TN liquid crystal display device is not suitable, in particular, for motion picture display of TV video, etc.
On the other hand, attention has been paid to an OCB liquid crystal display device as a liquid crystal display device that can enhance the viewing angle and improve the responsivity. In the OCB liquid crystal display device, a liquid crystal layer that is held between a pair of substrates includes liquid crystal molecules that can be oriented with a bend. Compared to the TN liquid crystal display device, the OCB liquid crystal display device has an improved responsivity that is higher by an order of magnitude. In addition, the OCB liquid crystal display device advantageously has a wider viewing angle since the effect of birefringence light, which passes through the liquid crystal layer, is optically self-compensated by the orientation state of liquid crystal molecules.
In the case where an image is displayed by the OCB liquid crystal display device, black may be displayed by blocking light at a time of, e.g. high voltage application and white may be displayed by passing light at a time of low voltage application, with the control of birefringence and in combination with a polarizer plate.
When a black image is displayed, a majority of liquid crystal molecules are oriented in an electric-field direction by the high voltage application (i.e. oriented in a normal direction to the substrates). However, liquid crystal molecules in the vicinity of the substrates are not oriented in the normal direction due to interactions with the orientation films. Consequently, light that travels through the liquid crystal layer is affected by a phase difference in a predetermined direction. Owing to the effect of phase difference, in the case of viewing the screen from a front-face side (i.e. in the normal direction to the substrate), the transmittance cannot sufficiently be reduced when a black image is displayed, and the contrast deteriorates.
To cope with this problem, a uniaxial phase plate, for instance, may be incorporated in the OCB liquid crystal display device. Thereby, the phase difference of the liquid crystal layer is compensated when a black image is displayed, and the transmittance can sufficiently be reduced, as is conventionally known. Besides, Jpn. Pat. Appln. KOKAI Publication No. 10-197862, for instance, discloses that phase plates including hybrid-aligned optically negative anisotropy elements are combined, whereby a black image with a sufficiently low transmittance is displayed or gray-level characteristics are compensated when the screen is obliquely viewed.
In the structure of the conventional OCB liquid crystal display device, coloring occurs when the screen is viewed from the front-face side. In particular, bluish coloring is recognized.